The present invention relates generally to the field of organic chemistry and more specifically to a method for the crosslinking of polymers, particularly to the crosslinking of polymers used in ionically conducting solids. These materials have applications in solid-state electrochemical devices (Shriver, D. F., et al., Chem. Eng. News 1985, 63(20), 42). Polymer-based complexes possess several advantages over semicristalline solids, and these advantages include ease of chemical modification and processability.
By way of background, lead-acid batteries suffer certain limitations in that they are far too heavy to be used as the primary energy storage device for many kinds of applications (i.e., for electrically powered vehicles). A number of high-energy-density alternate battery systems are known that would circumvent the problem of weight while having the same energy storage capacity.
NiCad (nickel-cadmium) batteries have superior battery characteristics for a very large number of applications. However, several disadvantages, including the adverse environmental effects of cachnium, initiated a search for more convenient solutions.
Liquid-electrolyte lithium batteries generally operate well in small scale applications but their disadvantages prevent them from becoming widely used. The disadvantages stem primarily from the steep vapor pressure-temperature curve for the solvent which causes the battery to leak electrolyte. Additional problems caused by formation of conductive dendrites (whiskers) which short the plates make the batteries prone to explosion due to high current flow and increased vapor pressure of the solvent. In spite of the disadvantages, lithium batteries have many clear advantages. It has been recognized that most of the problems with the liquid electrolyte type batteries could be overcome by developing a solid electrolyte lithium battery. A solid electrolyte lithium battery could eliminate the problems associated with high vapor pressure.
As a consequence intense research has been underway to find a solid electrolyte that would eliminate both the problems of explosions and dendrite growth and the subsequent shorting of the charge-discharge cycling. Most of the technical activity to date has centered on variants in the general class of polyethylene oxide (PEO)/lithium and polypropylene oxide(PPO)/lithium batteries. The major problem with those batteries has been the low conductivity of the solid electrolyte. The consequence is that a low conductivity limits a battery to low power densities. The way to get around this problem is to increase the conductivity of the solid electrolyte.
It has been shown that the polyorganophosphazenes like the high polymer poly[bis(2-(2-methoxyethoxy)ethoxy)phosphazene](MEEP) ##STR1## possess an ambient temperature ionic conductivity 2-3 orders of magnitude higher than poly(ethylene oxide), when each system is complexed with LiCF.sub.3 SO.sub.3 (Bennett, J. L., et al., Chem. Mater. 1989, 1, 14) or LiAlCl.sub.4 (Abraham, K. M., et al., Chem. Mater. 1991, 3, 339). However, a problem associated with the practical application of this polymer is its tendency to flow under light pressure. MEEP is a gelatinous substance that has been shown not to have adequate dimensional stability for use in a battery. Crosslinking is a well documented method for increasing structural integrity. As a consequence several methods of crosslinking MEEP have been developed. Previous studies have demonstrated that cross-linking of MEEP provides increased dimensional stability without reducing the ionic conductivity (Bennett, J. L., et al., Chem. Mater. 1989, 1, 14; Tonge J. S., et al., J. Electrochem. Soc. 1987, 134, 269). Prior methods for crosslinking of MEEP include chemical methods (Tonge J. S., et al., J. Electrochem. Soc. 1987, 134, 269) and radiation from the cobalt-60 source (Allcock, H. R., et al., Biomaterials 1988, 9, 509). Chemical crosslinking has that disadvantage of introducing impurities which might influence the conductivity. Chemical crosslinking requires the incorporation of a difunctional reagents, for example poly(ethylene glycol). Radiation crosslinking involves side-group coupling reactions. Moreover, the ability to crosslink the system with the salt already present allows much greater control over the materials properties and the shape of devices that employ the system. Although the gamma-radiation crosslinking method provides increased dimensional stability without reducing ionic conductivity, it is less appealing due to the fact of high cost of the source and elaborate precautions and accompanying expenses required with radioactive materials. The present invention overcomes some of the above-described disadvantages inherent with various solid electrolyte compositions and methods of the art. The invention presents methodology which permits rapid and convenient crosslinking of solid state battery electrolytes.
The present invention provides a method for crosslinking of the poly(organophosphazene) of the following formula: ##STR2## R=R.sub.1 ;R.noteq.R.sub.1 ##STR3## MEEP and other polymeric electrolytes by exposing the polymer to ultraviolet radiation. The UV source can be nothing more than a simple sun-tanning bulb.